This invention relates to memory systems which may be used in memory applications requiring high reliability. Specifically, the present invention relates to memory systems which utilize more memory elements than are needed to achieve the memory application's capacity requirements. More specifically, the present invention contains memory elements which may be brought "on-line" individually to permit the memory system to remain operational when failures occur.
The present invention provides a solution to a reliability problem. Reliability describes the likelihood of a component's failing within a given period of time under given operating conditions. It is well known in the art that duplication of memory elements can improve overall memory system reliability. However, inefficient duplication efforts fail to achieve as high a reliability as is possible given constraints of size, weight, or power.
On the other hand, the use of fewer parts typically raises overall system reliability because unused parts cannot fail. Therefore, an efficient utilization of memory elements permits the use of a fewer number of memory elements and raises system reliability. In addition, fewer parts require less power, and generate less heat. The power and heat effects each produce reliability and cost benefits.
One technique which tends to efficiently utilize memory in a bubble memory system having a plurality of bubble memory devices is described in U.S. Pat. No. 4,225,941 issued to Richard M. Moran. The Moran patent describes the use of a valid data memory external to the bubble devices. This valid data memory allows efficient utilization of bubble memory devices while preventing storage of data in blemished minor loops. While this technique tends to efficiently utilize bubble memory, it provides no memory element redundancy. In other words, all memory elements contained in the memory system are on-line whenever the memory system is active. Accordingly, any single point failure which occurs in the memory system or any change in bubble characteristics which causes the definition of blemished minor loops to change causes the Moran memory system to experience a failure.
Other memory systems incorporate redundant memory elements but bring back-up and secondary memory elements on-line in banks. A bank consists of a plurality of memory elements, so a failure in any one of the memory elements causes the entire bank to exhibit a failure. While this bank selection technique is conceptually simple, it may lead to a particularly inefficient use of memory because a failure in only one memory element causes the entire bank to be declared defective. Furthermore, the efficiency as applies to volume occupied decreases as the memory capacity of individual memory elements increases. Thus, this technique may be particularly unsuitable in memory systems using bubble memory for the memory elements. More memory elements are required than may be necessary, and the system reliability is less than may otherwise be possible.